In recent years, liquid crystal display devices or organic EL display devices have been required to have a high aperture ratio to achieve bright display or low power consumption. To improve the contrast ratio of the display by dividing pixels, these display devices typically use a black matrix formed by carbon dispersion or the like as a black color material in a photosensitive resin.
(Light Shielding Properties of Black Matrix)
The black matrix, which is disposed dividing pixels to ensure display contrast, is typically formed on a transparent substrate such as glass using a black resin with a large thickness of 1 μm (micrometer) or more so as to obtain high light shielding properties. The black resin in this case is obtained by dispersing a color material such as carbon pigment in a resin. The black matrix in a frame portion on four sides of a display area where pixels are arranged in a matrix pattern, i.e. the frame-like black matrix, is particularly required to have high light shielding properties, i.e., high optical density of 5 or more, or 6 or more in transmission measurement. Light transmitted from a backlight unit is likely to leak from the frame portion, and therefore the black matrix in the frame portion is required to have a higher optical density than that of the black matrix formed on the display area.
(Forming Black Matrix with Thin Lines)
In display devices for small-size mobile devices, such as cellular phones, displays are being formed with increasingly higher definition such as 200 ppi (pixels per inch) or more, or 300 ppi or more. Accompanying this, black matrixes are required to have thinner lines, in addition to the need to have high light shielding properties. If black matrixes achieve high definition, the pixel width unavoidably becomes smaller than 30 μm. It has been revealed that a smaller thickness of the black matrixes adversely affects the flatness of the color filters. In a high-definition display device having 300 ppi or more, the black matrix needs to have a line width of 4 μm or less.
For example, since black matrixes have high light shielding properties, it is difficult to stably manufacture a pattern of black matrixes with a thin line having a width of 4 μm or smaller, by photolithography. If the black matrix has light shielding properties, sufficient light does not enter in a thickness direction of the black matrix at light exposure. Therefore, the thin film that forms a black matrix easily peels off in a process of development or the like.
Moreover, from the viewpoint of alignment, it is very difficult to form a black matrix with a thin line width of 4 μm or less through two-step photolithography processes, i.e. to form it with two layers, for the purpose of improving light resistance. Forming a black matrix through two-step photolithography processes tends to cause variation of line width or display unevenness, due to alignment errors.
In typically used processing steps of color filters and the like, since a plurality of screens are formed on a large-size transparent substrate, an alignment margin of ±2 μm, for example, is needed in general. Therefore, it has been difficult to form a black matrix by two-step photolithography processes.
(Touch Sensing Function in Display Device)
A method of enabling direct input to a liquid crystal display device or an organic EL display device is sought to be achieved, for example by attaching a capacitive touch panel to the display device, or providing elements suitable for touch sensing at portions of the display device, the portions being in contact with a liquid crystal layer, or the like. The method of providing elements suitable for touch sensing is called an in-cell method. For example, the in-cell method is based on electrostatic capacitance, or a technique of using an optical sensor.
The technique based on electrostatic capacitance is often applied to touch sensing based on the in-cell technique which enables the display device to acquire input using a pointer such as a finger or a stylus pen. This method based on electrostatic capacitance needs several two-set electrode groups for sensing electrostatic capacitance, as disclosed in PTLs 1 to 5.